Miller calls Pentecostalism a complex social movement within Protestant Christianity, with many different strains, that emphasizes the gifts of the Holy Spirit — specifically, speaking in tongues as well as supernatural healing and other manifestations of the Holy Spirit.

It is the fastest-growing Christian movement in the world and ranks second to Catholicism in numbers, if one assembles all of the different factions that sometimes can be found inside Catholic and Protestant churches, said Miller, executive director of USC’s Center for Religion and Civic Culture.

Miller personally witnessed the tangible life-changing programs of those churches around the world. He observed that families often experienced upward social mobility because the money that had been used on alcohol or gambling was now available to educate one’s children or invest in a small business.

One of the biggest draws of the churches is their agenda to create social programs to improve people’s health or to spur economic development in the community.

“People are attracted when they see good works,” said Miller, who also holds an appointment in sociology.

That also surprised Miller, who had believed in the stereotype that Pentecostals were more interested in “saving souls rather than helping society.”

Miller also credits Pentecostalism’s popularity to its ability to adopt contemporary music, creating ecstatic worship services that are “the heart of the movement,” Miller said.

“They are very connected to the culture of the people,” Miller said. “The vast number are poor, but there is an emerging middle class. And their worship is vibrant. They use the musical idiom of the people.”

These services may sound a bit clinical when described in print, so Miller included a DVD with the book.

Miller captured footage of Pentecostal religious worship, testimony and social activism, and he includes interviews with Pentecostal pastors and leaders from around the world.

“Religion is not simply a matter of beliefs,” he said. “It is a whole body experience, especially for Pentecostals.”

Miller, a liberal Episcopalian, and Yamamori, an evangelical Christian, never planned to focus on Pentecostalism.

Miller and Yamamori were looking to study rapidly growing churches in urban environments of developing nations that also helped with social issues. They spent four years traveling the globe conducting extensive research in Africa, Asia, Latin America, South America and Europe.

And the churches that kept meeting the criteria in country after country were Pentecostal.

Pentecostalism first took hold just over 100 years ago in an abandoned church in Los Angeles on Azusa Street. The Pentecostals were set apart by their racial makeup, their openness to female leadership and their gifts of speaking in tongues.

The Azusa Street revival launched a missionary movement that sent thousands abroad to preach the gospel with little theological sophistication or ties to church bureaucracy.

Today Pentecostal growth in the United States is nearly flat compared to its explosion around the globe. And while many of the established Pentecostal religions such as the Assemblies of God are doing well outside of the United States, much of the international growth is coming from homegrown forms of this faith, Miller found.

The book was funded by Fieldstead and Co., a charitable organization of Howard and Roberta Ahmanson.